This invention is directed to an apparatus for high pressure injection of a sample into chromatography apparatus generally and into high pressure liquid chromatographic (HPLC) apparatus in particular. The design of this apparatus is such that the phenomenon of sample "tailing" is substantially eliminated. Thus, the apparatus is particularly useful for capillary-based chromatographic systems.
A typical prior art method of injecting a sample into pressure-driven chromatography systems, such as an HPLC apparatus, is illustrated schematically in FIG. 1. Here, a source of pressure, which in conventional chromatography systems is generally a piston or cam-driven pump, is initially, used to force a stream of buffer solution through a packed HPLC column. At the proper time, a sampling valve is turned admitting a portion of a sample into the flowing buffer solution stream and onto the HPLC column where the various components are separated and pass to a detector for analysis. However, for accurate analysis, particularly for complex samples, it is necessary to control precisely the opening and closing of the sampling valve to minimize flow-induced mixing and dead volume, all of which are very difficult to do.
Miniaturization of the chromatography apparatus offers several advantages including, improved efficiency, greater detection sensitivity, low solvent consumption, speed, and the need for only small quantities of sample (typically in the .mu.L range). In the extreme, complete microscale chromatography systems have been developed that fit into a single cm-size substrate. Examples of these systems can be found in U.S. Pat. No. 5,885,470 to Parce et al., U.S. Pat. No. 5,858,195 and International Application WO 96/04547 to Ramsey, and U.S. Pat. No. 5,571,410 to Swedberg et al. However, in microanalytical pressure-driven chromatography systems the problem of providing a sample having a well-defined volume is exacerbated. It has been found that it is extremely difficult to define accurately a sample volume injected into the chromatography column since the sample itself is quite small. This is generally a consequence of the fact that sampling valves suitable for pressure-driven microscale chromatography systems are either not available or are incapable of being opened and closed precisely enough to eliminate sample "tailing", flow-induced mixing and dead volume. Moreover, there can be changes in sample composition since faster migrating compounds will be introduced into the chromatography column preferentially. There have been many attempts to alleviate sample "tailing" problem inherent in pressure-driven microanalytical chromatography systems, none have been entirely successful.